Torque sensors for detecting torque exerted around a predetermined rotation axis have been widely used in a variety of transport machines and industrial, machines. For example, in Patent Document 1 given below, there is disclosed a torque sensor of a type in which mechanical deformation caused by exertion of torque is detected by a strain gauge. Further, in Patent Document 2, there is disclosed a sensor for detecting torque exerted on a shaft by forming a magnetostrictive film through plating on the shaft surface and measuring a change in magnetic properties of the magnetostrictive film. Still further, in Patent Document 3, there is disclosed a torque sensor of a type in which a magnetism generating part is provided at an end portion of a torsion bar, and a change in magnetic flux density of magnetism generated by the magnetism generating part is detected by use of a magnetic flux collecting ring. In Patent Document 4, there is disclosed a torque sensor of a type in which a large number of magnets are disposed in a cylindrical shape so that the N poles and the S poles are lined up alternately in the circumferential direction and magnetic fields generated by these magnets are detected.
On the other hand, there is also proposed a torque sensor in which the shape of an annular member is deformed by exertion of torque to detect electrically a mode of the deformation. For example, in Patent Document 5, there is disclosed a torque sensor for which a link mechanism for deforming in a radial direction the shape of an annular member by exertion of torque is prepared and in which a force applied in the radial direction is detected by a load sensor on the basis of deformation of the annular member. In Patent Document 6, there is disclosed a torque sensor in which strain gauges are used to detect an expansion-contraction state of each part of an annular member.
Further, for example, in Patent Documents 7 and 8, there is disclosed a method for using a capacitive element as a unit for electrically detecting displacement occurring at each part of a structural body. A capacitive element can be constituted of a pair of opposing electrodes, and a distance between both electrodes can be detected as a capacitance value. Therefore, it is suitable for a displacement detection unit provided on a sensor. Thus, in Patent Document 9, there is proposed a torque sensor in which the shape of an annular member is deformed by exertion of torque to detect displacement at each part resulting from the deformation by using a capacitive element.